The invention relates to a method of operating an internal combustion engine, which has at least two cylinder banks that can be controlled separately depending on the desired engine power output.
The document DE 196 11 363 C1 discloses a multi-cylinder internal-combustion piston engine having two cylinder banks which are both operative during full-load operation of the internal combustion engine, whereas one cylinder bank is shut down during part-load operation in order to save fuel and to improve the exhaust emission behavior. The internal combustion engine has a different dynamic behavior during part-load operation than it has during full-load operation. This has to be taken into account in the control of the internal combustion engine, in particular the throttle-valve control, the Lambda control or the knock control, and also in the determination of the engine-related control parameters, for example the ignition and injection timing parameters. For an optimal setting, therefore, it is necessary that specially adapted performance graphs are assigned to each mode of operation. In particular when changing over from full-load operation to part-load operation, or vice versa, care must be taken to ensure that controller instabilities are avoided.
The document DE 33 34 720 C2 discloses a control apparatus for cylinder shutdown by means of which a cylinder bank can be shut down only at steady-state operating points of the engine, whereby the risk of instabilities is reduced but the shutdown instant is restricted to steady-state operating points.
A further problem in internal combustion engines with bank-selective operation can occur as a result of asymmetric loading during full-load operation, which can be attributed to production tolerances. In this case, the performance data of the cylinder banks deviate from one another since the air supply, the ignition timing, the injection and/or the actuation of the gas exchange valves may differ for each cylinder bank. This results in non-uniform component loading in the cylinder banks and, in addition, in an impaired response behavior in particular under transient operating conditions. Primarily the smoothness of the engine is adversely affected.
It is the object of the invention to reduce the thermodynamic and mechanical loading in an internal combustion engine with bank-selective operating capability. Furthermore, the response behavior is to be improved.
In a method of operating an internal combustion engine which has at least two cylinder banks that can be controlled separately as a function of a desired load, wherein a desired pressure in the intake pipe of the internal combustion engine is determined and set by means of an air supply device for supplying combustion air to the cylinders of the internal combustion engine, at least one engine-related characteristic value which affects the behavior of the internal combustion engine is determined for each cylinder bank, and, in the event of a difference between the characteristic values of the cylinder banks, at least one cylinder bank is manipulated in such a way that the average value of the considered characteristic value corresponds to a desired average value in order to reduce the thermodynamic and mechanical loading of the internal combustion engine with bank-selective operating mode.
With the method according to the invention, at least two cylinder banks of the internal combustion engine can be controlled separately by determining a characteristic value of the internal combustion engine for each cylinder bank and, in the event of a difference between the characteristic values of the different cylinder banks, at least one cylinder bank is controlled in such a way that the average value of the considered characteristic value corresponds to a desired average value. The characteristic values considered are, especially the ignition timing, the beginning of fuel injection, the duration of fuel injection and/or the opening and closing curves of the gas exchange valves. As one of these characteristic values which can describe the thermodynamic behavior of the internal combustion engine is determined for each cylinder bank and the cylinders are so operated that a desired average value is achieved, differences in the cylinder banks of the internal combustion engine which, in particular, can be attributed to production tolerances are at least partly compensated for. As a result, an at least approximately uniform loading of the individual cylinder banks during part-load operation can be achieved. Non-uniform, asymmetrically distributed instances of loading between the cylinder banks are avoided.
As engine-related performance characteristics, which form the basis of the assessment of whether a behavior deviates from a standard, certain actual values of the internal combustion engine are preferably determined, either directly by measurement or indirectly from a correlation with a measured characteristic value, or a characteristic value of the internal combustion engine determined in some other way. The momentary injection times for each cylinder bank are determined as characteristic values. This provides for affording the advantage that the actual injection times are known from the energizing and de-energizing instants of the fuel injection elements, so that an additional measurement is unnecessary. The present injection times can be the beginning of injection and/or the duration of injection or the instant at which injection ends.
Advantageously, in the event of a difference between the characteristic quantities considered, the cylinder banks are set by manipulation of characteristic values which differ from those characteristic values which form the basis of the assessment of the behavior of each cylinder bank. In this case, it has proved to be expedient to provide gas exchange valves whose opening and closing curves can be set, in particular with regard to opening and closing instants, which can preferably be realized by using electromagnetically actuable gas exchange valves.
The function via which the cylinder banks are to be set advantageously comprises a differential element in which the difference between the characteristic quantities of two cylinder banks to be investigated is formed. This difference is subsequently fed to an integrator element in which numerical integration is carried out. The correction value is present at the integrator output and is to be supplied to one, or preferably to both, cylinder banks. By respective positive or negative signs, the correction value cylinder bank which has comparatively higher characteristic performance values is weakened, and conversely a cylinder bank with comparatively weaker characteristic performance values is strengthened. As a result of this procedure, the level of the considered characteristic performance values is lowered in one cylinder bank, whereas the level is raised in the other cylinder bank. In this embodiment, the desired average value is not calculated explicitly, rather level equalization is automatically established by virtue of the uniform raising and lowering of the considered characteristic performance values in the cylinder banks. The level is between the initial values of the considered characteristic performance values but not necessarily corresponding to the arithmetic mean of the initial values. It is unnecessary for the desired average value to be explicitly determined.
In order to obtain a smooth profile of values, it may be expedient to subject the considered characteristic performance values to low-pass filtering, in particular after the difference formation. The low-pass filtering has the effect of filtering at frequencies below the low-pass filter cut-off frequency.
In a preferred embodiment, the timing ignition and the fuel injection timing of each cylinder bank are determined as a function of the desired load, which can be determined as a desired value by driver specification, in particular the position of the accelerator pedal, but also as a function of further values such as control variables of a driving-dynamics stability program. This embodiment, which can also be referred to as predictive load control, is distinguished by a fast response behavior since the desired load acts as a pre-control value which directly forms the basis for the ignition timing and the fuel injection timing. The desired load advantageously forms the basis of both the control of the fuel side and the setting of the air side of the internal combustion engine. The air side adjustments can be made by manipulation of the gas exchange valves, and if appropriate alternatively or supplementary thereto also by the setting of a throttle valve in the intake tract.
Further advantages and features of the invention will become apparent from the following description of an embodiment thereof described on the basis of the accompanying drawings: